Photolithography is often used in the manufacture of semiconductors. A reticle having a circuit pattern thereon is projected onto a photosensitive substrate such as a wafer. The wafer is subsequently processed, forming patterns thereon. This process is generally repeated several times, forming multiple layers. As the art develops and the element sizes of the semiconductor devices become smaller, there is a need for improved illumination sources for projecting the image of a reticle onto a photosensitive substrate. Accordingly, there have been many illumination systems disclosed in an effort to enhance the imaging of a reticle onto a photosensitive substrate. One such illumination system is disclosed in U.S. Pat. No. 5,296,892 entitled "Illuminating Apparatus and Projection Exposure Apparatus Provided With Such Illumination Apparatus" issuing to Mori on Mar. 22, 1994, which is herein incorporated by reference. Therein disclosed is an illumination system having an optical integrator or fly's eye lens positioned next to a condenser. The optical integrator or fly's eye lens is designed to be replaceable so that the numerical aperture on the emission side of the illumination system can be varied. Another illumination system is disclosed in U.S. Pat. No. 5,357,312 entitled "Illuminating System In Exposure Apparatus For Photolithography" issuing to Tounai on Oct. 18, 1994, which is herein incorporated by reference. Therein disclosed is an illumination system that desirably shapes a uniformalized light beam reducing loss of illuminating light by blockage of an aperture diaphragm. Additionally, the cross sectional shape of the light beam can be rendered annular. Yet another illumination system is disclosed in U.S. Pat. No. 5,329,336 entitled "Exposure Method and Apparatus" issuing to Hirano et al. on Jul. 12, 1994. Therein disclosed is an exposure apparatus having a detector that is compensated for due to coherence changes in the exposure apparatus. Additionally, the concept of zone and modified light source illumination is disclosed for the purpose of optimizing the image of a reticle pattern. Accordingly, there is a need to provide different illumination characteristics for different reticle geometries or patterns. However, due to the complexity of illumination sources and the difficulty of modifying them for a specific reticle pattern or geometry, often it is difficult to provide an optimized illumination pattern for a specific reticle. As a result, some portions of a reticle may be illuminated adequately while others are not. This may result in different imaging results on different portions of a wafer. Therefore, there is a need for an illumination system that optimizes the exposure of a photosensitive substrate irrespective of the different reticle patterns or geometries contained thereon.